Known methods of detecting and counting instantaneous variations concern detecting and counting the dislocations caused by the overlap between two consecutive flat objects on a conveyor. They are consequently related to and limited to applications of counting continuously travelling flat objects that overlap one another on a conveyor.
Several methods and apparatuses based on different operating principles are known for use in counting continuously travelling flat objects overlapping one another on a conveyor. For example, there are mechanical or electromechanical counters that always require physical contact with the travelling articles, and there are optoelectronic counters which operate remotely without physical contact.
U.S. Pat. Nos. 3,969,993, 4,091,269, and 4,139,765 describe several methods and apparatuses for counting continuously travelling flat objects by means of physical contact. The reliability and accuracy of these methods and apparatuses depend, amongst other things, on the thickness of the objects, and consequently on the size of the dislocations, on the distance between two dislocations, on the speed of travel, and on the amplitude of conveyor vibration. Experience shows that these methods and apparatuses, even when operating under optimum conditions, are not satisfactory when the objects are thin and the speed of travel is very high. In addition, these apparatuses are subjected to the wear inherent to mechanical systems.
Several patents relate to optoelectronic detection and counting without physical contact. By way of example, mention may be made of the following patents: JP No. 770, GB 8410493, U.S. Pat. No. 4,450,352, and U.S. Pat. No. 4,771,443. The methods and apparatuses described in these patents share the common feature of using light reflected on the surface of continuously travelling objects for the purpose of detecting the passage of a dislocation and causing it to be counted. In spite of the considerable efforts that have been made to improve the reliability of the methods and apparatuses by making use of microprocessors in order to process the reflected light more reliably, the results obtained do not give entire satisfaction. The main parameters which disturb these methods and apparatuses are the surface state of the object (mat or brilliant), its color which reflects radiation more or less well, and vibration of the conveyor which may sometimes vibrate through an amplitude which is greater than the thickness of the object.
All of these parameters may influence the detection results and give rise to errors depending on the principle used.
All of the methods and apparatuses mentioned above provide an output signal which is directly proportional to variations in the function Y=f(X). Given that the value of Y corresponds to the thickness of the object or the set of objects, the output signal is consequently directly proportional to the thickness of continuously travelling objects. This implies that the apparatus needs adjusting whenever there is a change in the goods being produced in order to take account of the differences between the parameters applicable to the new run and the parameters applicable to the preceding run.
An object of the invention is to provide a method and apparatus enabling instantaneous variations, and only instantaneous variations, to be detected and counted, by scanning the profile of a continuously travelling object or set of overlapping objects on a conveyor. The method and the apparatus which provide an output signal directly proportional to the value of the derivative Y' rather than the value of the function Y mitigate the observed drawbacks in that operation is independent of the thickness of the travelling objects, of their surface state, of their speed of travel, of the amplitude of any vibrations to which they may be subjected as they travel, and of the distance between two consecutive instantaneous variations.